Gas turbine engines are typically employed to power aircraft. Typically a gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven directly off an additional lower pressure turbine in the engine core.
The fan comprises an array of radially extending fan blades mounted on a rotor and will usually provide, in current high bypass gas turbine engines, around seventy-five percent of the overall thrust generated by the gas turbine engine. The remaining portion of air from the fan is ingested by the engine core and is further compressed, combusted, accelerated and exhausted through a nozzle. The engine core exhaust mixes with the remaining portion of relatively high-volume, low-velocity air bypassing the engine core.
The fan blades and/or a casing that surrounds the fan may be manufactured from metallic and/or composite non-metallic materials. Generally fan blades include a composite body and a metallic leading edge and tip.
Composite components are often laminate structures that include a plurality of plies. Each ply generally includes a reinforcing fibres (e.g. high strength or high stiffness fibres) embedded in a matrix, e.g. a plastic matrix material. The matrix material of adjacent stacked plies is bonded together to build the composite component. The matrix material is weaker than the fibre material and as such the bond between stacked plies can form a point of weakness. This means that a primary failure mechanism of concern for composite materials is delamination.
Delamination for example of a fan blade may occur in the event of an impact by a foreign object such as a bird strike.
To reduce the risk of delamination of a composite component through thickness reinforcement can be used. One type of through thickness reinforcement is z-pinning. A component that has been z-pinned includes a plurality of rods (known as z-pins) extending through the thickness of the component in a direction transverse to the general direction of the plies. Z-pins are generally made of a metallic or composite material and typically have a diameter ranging from or equal to approximately 0.3 mm to 0.5 mm. Often, composite z-pins are manufactured by pultrusion of a carbon fibre tow impregnated by a thermoset resin. The z-pins of a composite component exert a bridging force on the plies to hold the plies in position relative to each other, this reduces opening of inter-laminar cracks (known as mode I failure) and sliding displacements of inter-laminar cracks (known as mode II failure).
Although the z-pins can substantially arrest crack propagation, it is often not possible to eliminate crack propagation in a composite component. Accordingly, it would be useful to detect when delamination has occurred, and the extent and/or location of such delamination so as to indicate when a component needs repair or replacement.